1. Field of the Invention
This invention relates to an ink feeding method for a printing machine, for controlling an ink feeding rate by measuring color density of prints produced.
2. Description of the Related Art
A printing machine has an ink feeding apparatus for adjusting the rate of feeding ink to ink rollers. The ink feeding apparatus includes a plurality of ink keys juxtaposed in a direction perpendicular to a direction in which printing paper is transported during a printing operation. The rate of feeding ink to the ink rollers is adjusted by varying the opening degree of each ink key. In this way, the rate of feeding ink ultimately to a printing plate is adjusted.
The printing plate has areas called color patches formed in positions corresponding to the respective ink keys. The color density of the color patches actually printed on the printing paper is measured with a densitometer to adjust the opening degree of each ink key.
When printing with such a printing machine, the color density of prints may not agree with a predetermined value immediately after start of a printing operation even though the ink keys in the ink feeding apparatus have a proper opening degree. In such a case, when the color density of prints is measured and the ink feeding rate is automatically controlled, the opening degree of the ink keys, even though proper, is further adjusted in an opening direction.
Since numerous ink rollers are used in such a printing machine, a predetermined time is taken until an adjustment of the opening degree of each ink key is reflected in the rate of feeding ink to printing paper. Thus, when the ink feeding rate is automatically controlled by measuring the color density of prints immediately after adjusting the opening degree of the ink keys, the opening degree of the ink keys is further adjusted even though the opening degree is proper.
The rate of feeding dampening water to the printing plate influences the rate of feeding ink to the printing plate. Thus, when the ink feeding rate is automatically controlled by measuring the color density of prints immediately after adjusting the rate of feeding dampening water to the printing plate, the opening degree of the ink keys is further adjusted even though the opening degree is proper.
An adjustment of the opening degree of the ink keys, therefore, is prohibited immediately after start of a printing operation, or after an adjustment is made of the ink or water feeding rate, until a predetermined number of sheets are printed or until lapse of a fixed time.
However, where a long time is set for the above prohibition, the ink feeding rate cannot be controlled quickly. This presents a problem of taking a long time before the color density of actual prints settles at a target value.
On the other hand, when the opening degree of the ink keys is varied excessively to control the ink feeding rate quickly, a gross overshooting will occur before the color density of prints settles at a target value.
Applicant has proposed an ink feeding method for a printing machine, for enabling the color density of prints to settle at a target value quickly without causing a gross overshooting. This method comprises a color density measuring step for measuring color density of prints at selected times, a color density gradient computing step for computing, based on the color density of prints measured in the color density measuring step, a color density gradient representing a rate of variation in the color density of prints occurring with an increase in the number of prints, an expected color density computing step for computing, based on the color density gradient computed in the color density gradient computing step, an expected color density of prints occurring after a predetermined number of prints are made, and an ink feeding rate controlling step for controlling the ink feeding rate based on the expected color density of prints computed in the expected color density computing step and a target color density of prints (see Japanese Unexamined Patent Publication No. 2003-334927).
The ink feeding method for a printing machine described in Japanese Unexamined Patent Publication No. 2003-334927 is excellent in terms of enabling the color density of prints to settle at a target value quickly without causing a gross overshooting. However, this method has a disadvantage of requiring time and skill in adjusting parameters relating to the control of the ink feeding rate, such as the number of prints to be made to serve as a basis for computing an expected density each time, and a control coefficient for use in controlling the ink feeding rate.